The invention relates to a device for remotely determining the spatial position of a rotatable object. The movement of the object relative to a fixed trihedral reference frame is resolved into rotational components.
Such positional determinations are especially useful in applications where variations in the orientation of the head of a pilot of an aircraft relative to the cockpit should be followed, for example in order to obtain an indication of said movements for a subsequent study or in order to provide automatic control in accordance with changes in the pilot's line of sight. For this type of applications the fixed reference frame is represented by the cockpit and the movable object in the space of said cockpit is generally constituted by the helmet on the pilot's head, which helmet moves in accordance with the direction of visual observation of the pilot.
Devices are known which have been designed in order to enable the angular position of the pilot's line of sight to be determined by remote measuring means, without any mechanical linkage between the pilot's helmet and the cockpit and employing polarized light. Such a device is described in U.S. Pat. No. 3,867,629. The pilot's helmet is provided with a reflector and a polarizer, which together constitute a source of polarized light, the direction of the polarizing vector varying with the angular position of the helmet relative to the axis of propagation of the beam. Said beam is directed at a device for detecting the angular position of the polarizing vector, which device comprises a rotating analyser and an optical sensor. Said sensor is sensitive to the luminous intensity of the beam which it receives via the analyser, which analyzer is mounted on a rotary disc which is perpendicular to the axis of propagation of the beam. This detection device is stationary with respect to the object being observed. For the specific use considered the disc carrying the analyser is rotatably mounted on the cockpit and the optical sensor is rigidly connected to said cockpit behind the analyser. This means that the direction of the beam should remain fixed. Measuring is effected by determining the angle through which the disc has rotated from an angular reference position when a maximum light transmission through the analyser indicates the coincidence of its direction of polarization with that of the light beam from the helmet, said last-mentioned direction, being related, as already set forth, to the angular position of the helmet relative to the axis of propagation of the beam.
British Patent Specification No. 1,045,994 discloses how, in a device of the same type, variations in luminous intensity as a function of the angular distance existing at any instant between the analyser and the polarizer may be used in a differential measurement. A polariser mounted on the pilot's helmet cooperates with a system comprising a plurality of analysers mounted on one and the same rotary disc with differently oriented polarizing axes. The system is made to rotate around the light beam, its angular position being referred to the cockpit until the luminous intensities of the beams transmitted by the differently oriented analysers are equal. As in the system described in the previously mentioned U.S. patent, observation is limited to rotational movements of the helmet about a single axis, which is fixed relative to the cockpit, in accordance with which axis the light beam used for the measurements propagates.